[Delegations prereq] Make snapshot manifest list dynamic

Splitting up #175

repo.go

@@ -28,11 +28,6 @@ var topLevelMetadata = []string{
 	"timestamp.json",
 }
 
-var snapshotMetadata = []string{
-	"root.json",
-	"targets.json",
-}
-
 // TargetsWalkFunc is a function of a target path name and a target payload used to
 // execute some function on each staged target file. For example, it may normalize path
 // names and generate target file metadata with additional custom metadata.
@@ -812,6 +807,12 @@ func (r *Repo) Snapshot() error {
 	return r.SnapshotWithExpires(data.DefaultExpires("snapshot"))
 }
 
+func (r *Repo) snapshotManifests() []string {
+	// Note: root pinning is not supported in Spec 1.0.19.
+	// root.json might need to be removed.
+	return []string{"root.json", "targets.json"}
+}
+
 func (r *Repo) SnapshotWithExpires(expires time.Time) error {
 	if !validExpires(expires) {
 		return ErrInvalidExpires{expires}
@@ -826,12 +827,12 @@ func (r *Repo) SnapshotWithExpires(expires time.Time) error {
 		return err
 	}
 
-	for _, name := range snapshotMetadata {
-		if err := r.verifySignature(name, db); err != nil {
+	for _, manifestName := range r.snapshotManifests() {
+		if err := r.verifySignature(manifestName, db); err != nil {
 			return err
 		}
 		var err error
-		snapshot.Meta[name], err = r.snapshotFileMeta(name)
+		snapshot.Meta[manifestName], err = r.snapshotFileMeta(manifestName)
 		if err != nil {
 			return err
 		}
@@ -958,7 +959,7 @@ func (r *Repo) Commit() error {
 	if err != nil {
 		return err
 	}
-	for _, name := range snapshotMetadata {
+	for _, name := range r.snapshotManifests() {
 		expected, ok := snapshot.Meta[name]
 		if !ok {
 			return fmt.Errorf("tuf: snapshot.json missing hash for %s", name)